This invention relates to a method of packaging, cooling, and storing fresh produce. While applicable to many crops it is particularly useful for table grapes.
The miracle of modern storage and transportation systems has made fresh fruit and produce of the highest quality available year around. Even delicate offseason produce such as raspberries and strawberries find their way from growers in South America to tables in the United States while still in prime condition. Apples and pears are crisp and juicy six to nine months after they were picked. In part this is due to controlled atmosphere cold storage of fruit in which the oxygen level is reduced and carbon dioxide level is elevated. Periodic fumigation to prevent botrytis growth is routinely required of other products. Storage requirements for the particular product, at least in part, dictates the type of packaging used.
If table grapes might be used as an example, approximately 2 pounds (0.9 kg) is usually field packaged in individual ventilated bags. Most usually six to twelve bags are then placed in a single layer in a corrugated shipping container. The containers have side perforations to permit fumigation while in storage. Sulfur dioxide is the usual fumigant and treatment is typically on a weekly basis during storage.
One type of packaging for products requiring fumigation employs complex film enclosures having differing permeability to various gases. Floyd et al., U.S. Pat. No. 5,458,899; Bailey, U.S. Pat. No. 5,565,230; and Hayashi et al., U.S. Pat. No. 5,698,249; might be exemplary of this approach. Porchia et al., U.S. Pat. No. 5,492,705; and Doyle, U.S. Pat. No. 5,171,593; both describe bags having a multiplicity of microperforations which allow the product to breathe so that localized condensation and weight loss are minimized.
However, bags with overall perforations have not proved satisfactory for many products. If the perforations of a bag are in contact with the kraft paper surface of a corrugated shipping container, severe moisture loss of the contents will occur. The paper acts as a highly efficient desiccant to withdraw moisture from the product in the bag. To again use table grapes as an example, moisture loss from initial packaging until they appear on the grocer""s shelves must be less than about 2%, otherwise browning and deterioration will begin to occur.
Two requirements must be met in produce bags for fumigated packaging. They must be tight enough so that product moisture is retained yet open enough so that fumigant can freely enter. These would appear to be almost mutually exclusive requirements. However, Muise et al., in U.S. Pat. No. 5,919,504, describe a produce bag having controlled ventilation properties that has successfully met both demands. This patent, commonly assigned with the present application, is hereby incorporated in its entirety by reference.
As was noted, table grapes are usually bagged and boxed in the field. They are then palletized and transported to a cold storage warehouse. Typically they are prechilled before entering storage and may receive an immediate fumigation. While several methods of prechilling are used, a so-called pressure cooler is the most common method. A number of pallets, usually about six or more, are placed end-to-end in a row with a second similar parallel row displaced a short distance so as to form a channel between them. Either a single or double height of pallets may be used, depending on the capacity of the system. The tops and one end of the pallet assembly are covered with a tarpaulin. The other end is maintained open. Cold air is forced in from the sides and flows through the boxed produce, down the channel, and out the open end where it is again cooled and recycled. Produce temperature is preferably reduced to the 0xc2x0-1xc2x0 C. range in the pressure cooler whereupon the pallets are then transported to the cold storage warehouse kept at about the same temperature. Cold storage warehouses are most often maintained at about 90% relative humidity (RH) to reduce moisture loss of the produce.
At peak harvest times and in hot weather the available pressure cooler system may not have sufficient capacity to bring the entire pick down to 0xc2x0-1xc2x0 C. Higher prechilling temperatures must be tolerated; e,g., about 4xc2x0-8xc2x0 C., in order to treat the entire pick and avoid spoilage before the loaded pallets are placed in the warehouse.
One significant problem associated with high humidity cold storage conditions is loss of strength of the corrugated shipping containers. Edge compression strength of a container at 90% RH is only about half of that at 50% RH. This loss of strength can result in container failure and damage to the contents during storage and shipping. It is often compensated for by the use of heavier, and more expensive, containers. Muise et al., in U.S. Pat. No. 5,619,841, discuss this problem in detail. They describe a packaging system that uses an unperforated plastic bag within the corrugated container. The contained produce is sprayed with a predetermined amount of water and vacuum chilled. The bag prevents moisture loss but is not amenable to fumigation. However, this packaging system is amenable to storage at reduced relative humidity without loss of produce moisture. This patent is also commonly assigned with the present application and is included by reference.
The present invention particularly directed to a packaging and storage system for field produce that enables both reduced humidity storage and periodic fumigation
The present invention is a method for packing and storage of field produce. It is particularly directed to the problems of prevention of moisture loss from the produce while still allowing periodic fumigation, where this might be necessary, yet permitting a reduced relative humidity storage environment. The method also normally permits reduced prechilling time. In many circumstances the method allows some crops to be directly placed in the cold storage warehouse without any prechilling.
A key to the practice of the method is the use of ventilated bags designed to prevent moisture loss while allowing access to fungicidal fumigation gases. The produce is normally put into the bags in the field. These, in turn, are given a casual closure and appropriately placed in corrugated shipping containers which are then palletized. xe2x80x9cCasual closurexe2x80x9d is defined here as the open end of the bag being unsealed and simply loosely folded or rolled to contain the contents. The pallets are then directed to a cold storage warehouse where the temperature is held in the range of about xe2x88x921xc2x0-2.5xc2x0 C. and the relative humidity is maintained below about 80%. Preferably the relative humidity is held in the range of about 65-75%, most preferably about 70% and the temperature in the range of about 0xc2x0-1xc2x0 C.
The palletized packaged produce may be fully prechilled before placing in cold storage. Alternatively, for some crops, table grapes being an example, it is usually sufficient to only partially prechill the contents to a temperature in the range of about 4xc2x0-7xc2x0 C. (39xc2x0-45xc2x0 F.). In the case of table grapes it has also been discovered that they may be placed directly in the cold storage warehouse without any prechilling without quality deterioration due to moisture loss or fungal growth. The ventilated bags in which the produce is initially placed are key to the success of the reduced humidity storage method. The precise pattern of ventilation will depend somewhat on the crop being packaged. However, bags having only one side ventilated using perforations having an open area of about 0.35-2.0% of the ventilated surface have been found to be extremely useful. Preferably, the open area is in the range of about 0.4-1.0% of the ventilated surface. Ventilation is accomplished by placing a multiplicity of small holes having diameters in the range of about 3-5 mm (0.12-0.20 inches) in the bags, preferably about 2.5-4.6 mm (0.10-0.18 inches) in diameter.
It is necessary that the ventilation holes be only on one side of the bag.
Specifically, they should be on the side of the bag that will be on the top when the bags are placed in their corrugated shipping container. The reason for this is that kraft paper acts as an efficient desiccant. Where the ventilation holes are in contact with the corrugated container surface the effect is to cause unacceptable moisture loss from the bag contents.
It is an object of the invention to provide a method of reduced humidity cold storage for field produce that will control moisture loss but still permit fumigation.
It is another object to provide a method than enables the produce to be entered into cold storage with out precooling or with reduced precooling.
It is a further object to provide a method of cold storage that will reduce loss of shipping container strength due to moisture pickup.
These and many other objects will become readily apparent upon reading the following detailed description taken in conjunction with the drawings.